Massive stars are essentially the driving force of the entire universe – in terms of energy, momentum, and
baryonic cycling through galaxies – over cosmic time. Without them the present universe would simply not exist.
They are key agents behind the chemical enrichment of the Universe, which has allowed the formation of the
heavy elements necessary for the formation of rocky planetary bodies such as our Solar System, complex
organic chemistry, and as such life itself. Indeed, current estimates show that everything on Earth was formed
by several hundred to thousands of massive stellar evolutionary generations early in the Universe.
Despite their importance and remarkable progress made by the Astrophysical community in the understanding
of their physical properties along the interconnected processes that drive their end-to-end life-cycle, there are
still many critical questions which remain open. This is a consequence of their complex formation process
(initialised by the highly turbulent, magnetised fragmentation of their host molecular clouds), and subsequent
evolution (which is continuously affected by an intricate network of physical processes). In brief, the whole endto-
end life-cycle of these extreme stellar objects is known to be dominated by (1) their initial mass, which is
intimately linked to the star formation process, (2) the large amount of angular momentum remaining in the
star once it has reached the Zero Age Main Sequence, and how this is shared between their cores and surfaces
along their evolution, (3) mass transfer processes occurring in the high percentage of stars of this type born in
binary and multiple systems, and (4) the strong -- sometimes steady, sometimes eruptive – mass loss events
driven by the interaction between the stellar radiation and the outer layers of these stars. Overall, this leads to
a still not fully understood diversity of possible final fates (represented by a variety of core-collapse supernovae
events) and end products (including isolated neutron stars and black holes, and binary systems comprising two
of these stellar corpses, which can eventually lead to a gravitational wave event if they merge).
Ensuring advance in this field hence requires the work and close interaction of researchers covering
different domains of expertise, including stellar atmosphere modelling, stellar structure and evolution
modelling, quantitative spectroscopy in different spectral windows and different stages of evolution, as well as
the investigation (both observationally and from the modelling point of view) of the star formation process and
the connection between massive stellar evolution and the associated end-products.